THE SENSORY RESPONSES OF CHITON 159 



for a variety of investigational purposes. Little has been known, 

 however, about the activities of placophorans, and no experi- 

 mental study has hitherto been made as to the nature of their 

 somewhat remarkable sensory organs. In the following pages 

 we shall be concerned with the behavior of C. tuberculatus and 

 with the several ways in which its responses to sensory activation 

 may be regarded as of general theoretic interest. 



Although it would be unwise to place upon the fact too great 

 an emphasis, in relation to the present study, it is nevertheless 

 well to recall that the Placophora are usually admitted to rank 

 as 'generalized,' or even 'primitive,' mollusks. In their marked 

 bilateral symmetry, their somewhat diagrammatic organization, 

 and the primitive aspect of their development (Heath, '99), 

 the Placophora are believed to display features of an ancient and 

 generalized character — which is not, however, fully supported by 

 the facts of their known fossil record, although animals clearly 

 of the chiton type are known from later Ordovician times onward. 

 We need not be led to expect that the primitive organization of 

 the chitons should necessarily determine the relative complexity 

 of their behavior as contrasted wdth that of other mollusks. The 

 very fact of their greater antiquity, apparently confirmed by the 

 morphological evidence, allows all the more opportunity for the 

 possible acquisition, among the chitons, of special features of 

 their own.- From this standpoint little can be gained through 

 the deliberate consideration of the behavior of Chiton in the light 

 of its morphological approximation to the 'ancestral mollusk;' on 



animal with which we have worked. In superficial characters it agrees with 

 the usual diagnosis of C. tuberculatus (Dall and Simpson, '01). with which C. 

 squamosis was for a long time considered to be identical. Specimens have been 

 identified for us as C. tuberculatus by Mr. W. F. Clapp, of the Museum of Com- 

 parative Zoology, to whom we are indebted for this and numerous other kind- 

 nesses. In external appearance this chiton is very variable, but it is undoubt- 

 edly the only species at all common at Bermuda. In three years' collecting by 

 one of us (W. J. C.) only a single Acanthopleura has been seen, although Ischno- 

 chiton purpurascens is fairly abundant, and Acanthochites and Tonicia less so. 

 Thus the chiton fauna of Bermuda differs much from that of the more southern 

 portions of the West Indian faunal region, where Acanthopleura is said to be 

 the commonest type, although four or five species of chiton proper, including 

 C. tuberculatus, are well known from Porto Rico and other stations (W. J. C). 



